Origin of dolomitized rocks of the Lower Cretaceous Bayanhua Group in Bayindulan Sag, Erlian Basin, Northern China

The Bayindulan Sag, located in Erlian Basin, Northern China, is characterized by pervasive dolomitization of Lower Cretaceous Bayanhua Group, Aershan Member to Tenggeer Member. Based on the analysis of core and well logging,... [ view full abstract ]

The Bayindulan Sag, located in Erlian Basin, Northern China, is characterized by pervasive dolomitization of Lower Cretaceous Bayanhua Group, Aershan Member to Tenggeer Member. Based on the analysis of core and well logging, it shows that, the strata from upper part of Aershan Member to lower part of Tenggeer Member has similar lithological characteristics and mainly consists of mudstones and siltstones with thinly-bedded tuffites. The thickness of per bedded tuffites ranges from 2m to 10m. Meanwhile, the upper part of Aershan Member to lower part of Tenggeer Member rocks have been widely dolomitized in varying degrees, which results in the formation of dolomitized rocks such as dolomitized mudstones, dolomitized siltstones, dolomitized sandstones, dolomitized tuffites, etc. The study on origin of dolomitization and controlling factors of dolomitized rocks distribution has important practical significance about further hydrocarbon exploration.
Based on petrography with geochemistry analyses, mud-crystal dolomites, silt-crystal dolomites and fine-crystal dolomites are observed, and the former two types are dominant. Meanwhile, a variety of dolomite textures such as scattered, blocky, Joined and annular textures have been observed. These dolomites have strongly depleted oxygen isotopic values (average=-10.5‰ PDB), carbon isotopic values (average= 5.4‰ PDB), and Sr values(average=370 ppm) as well as the Sr/Ba ratios (average=0.52). The obvious depletion is as a result of the increase in temperature with progressive burial, and the high carbon isotopic content of dolomites maybe result from the involvement of CO2 probably generated by bio-methanation in the dolomitization process. The Sr and Sr/Ba values both show that dolomitized rocks probably deposited in freshwater setting. Taking the petrographic and geochemical data into consideration, the results shows that the dolomites have formed in a freshwater environments during deep burial depths at relatively high temperature. Petrographic observations and mineralogy analyses suggest that dolomitization is closely associated with tuffaceous materials, the Mg comes mainly from the dissolution of tuffaceous materials and dolomitization is most actually driven by tuffaceous materials alteration at elevated temperatures. The contents of tuffaceous materials and the amount of organic matters play crucial roles in controlling petrographical characteristics and distribution rules of dolomitized rocks.